Case studies in engineering failure analysis最新文献

In mineral processing industries vibrating screens operate under high structural loading and continuous vibrations. In this regard, this may result in high strain rates, which may often lead to structural failure or damage to the screen. In order to lessen the possibility of failure occurring, theories and techniques for analyzing machine structures are investigated and applied to perform a sensitivity study of a newly developed vibrating screen. Structural strength and stability of a vibrating screen is essential to insure that failure doesn’t occur during production. In this paper a finite element analysis (FEA) on a reconfigurable vibrating screen (RVS) is carried out to determine whether the structure will perform as desired under extreme working conditions at the different configurations of 305 mm × 610 mm, 305 mm × 1220 mm and 610 mm × 1220 mm. This process is aimed at eliminating unplanned shutdowns and minimizes maintenance cost of the equipment. Each component of a screen structure is analyzed separately, stress and displacement parameters are determined based on dynamic analysis. In addition, a modal analysis was carried out for the first three (3) modes at frequency f of 18.756 Hz, 32.676 Hz and 39.619 Hz respectively. The results from the analysis showed weak points on the side plates of screen structure. Further improvements were incorporated to effectively optimize the RVS structure after undergoing an industrial investigation of similar machines.

Rolling bearing is an important and fragile component in the wind turbine transmission system. The failure of rolling bearing is one of the highest risk events which may result in unexpected economic loss. To give a proper condition assessment of rolling bearing, especially for early fault detection, is of great importance and become an urgent issue to the wind energy industry. In this paper, sample entropy is studied through the field data of wind turbine transmission system measured from Lu Nan Wind Farm in China. Compared with several frequently used statistical indicators, sample entropy features advantages in detecting and evaluating the progress of the early faults of the rolling bearing. The studies show that the sample entropy is an effective and practical tool for condition monitoring of rolling bearing for a wind turbine transmission system.

The failure analysis of a dissimilar weld in a heat exchanger has been conducted. Within hours of being placed in service, the circumferential weld joining the carbon steel shell to the duplex stainless steel tubesheet experienced partial cracking as H₂S was being introduced into the exchanger. The cracking of the weld was determined to be associated with sulfide-stress corrosion cracking facilitated by high weld hardness levels and local dilution of chemistry in the weld.

Gas turbines casings are susceptible to cracking at the edge of eccentric pin hole, which is the most likely position for crack initiation and propagation. This paper describes the improvement of transient thermal fatigue crack propagation life of gas turbines casings through the application of additional holes. The crack position and direction was determined using non-destructive tests. A series of finite element patterns were developed and tested in ASTM-A395 elastic perfectly-plastic ductile cast iron. The effect of arrangement of additional holes on transient thermal fatigue behavior of gas turbines casings containing hole edge cracks was investigated. ABAQUS finite element package and Zencrack fracture mechanics code were used for modeling. The effect of the reduction of transient thermal stress distribution around the eccentric pin hole on the transient thermal fatigue crack propagation life of the gas turbines casings was discussed. The result shows that transient thermal fatigue crack propagation life could be extended by applying additional holes of larger diameter and decreased by increasing the vertical distance, angle, and distance between the eccentric pin hole and the additional holes. The results from the numerical predictions were compared with experimental data.

Failure investigation was done on a 321 stainless steel charge heater tube which failed in a refinery unit processing heavy crude oil. Crude oil was the charge in the radiant and convection sections; while saturated stripping steam is present in convection section. After a leak was detected, visual inspection revealed that nine convection tubes had black oil/coke deposits on their external surfaces. The deposits were seen on the first three rows of tubes. When one of the tubes was lightly ground at the black colored area, a circumferential crack was visually observed. The investigation revealed that long-term aging, coupled with localized deposition of salts and coke from the heavy crude led to sensitization of the tube surface layers. This in turn resulted in sulphidation of the internal surface grain boundaries, formation of grooves, and cracking of the material. Thus, cracking was intergranular in nature in the initial stage, but became transgranular at later stages. It was concluded that cracking was due to chloride stress corrosion cracking catalyzed by the presence of sulphur-bearing species. It was recommended that the desalter operation be improved and frequent decoking and scale removal be carried out, with emphasis on the convection section at the refinery.

Failure analysis of the boiler water-wall tube is presented in this work. In order to examine the causes of failure, various techniques including visual inspection, chemical analysis, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy were carried out. Tube wall thickness measurements were performed on the ruptured tube. The fire-facing side of the tube was observed to have experienced significant wall thinning. The composition of the matrix material of the tube meets the requirements of the relevant standards. Microscopic examinations showed that the spheroidization of pearlite is not very obvious. The failure mechanism is identified as a result of the significant localized wall thinning of the boiler water-wall tube due to oxidation.

This paper presents the failure analysis of carbon steel roller shaft of continuous pad steam machine used in textile industry. The fracture position was located at a stepped diameter. The failed component was the shaft made of carbon steel AISI 1040. Standard procedure for failure analysis was employed in this investigation. Visual examination, chemical analysis, hardness and tensile strength measurements, microstructural characterization, fractography analysis by Scanning Electron Microscopy (SEM) and Finite Element Analysis (FEA) were used for the failure analysis. Using this failure analysis approach, we pinpointed the root cause of failure and developed a means of solving this type of failure in the future. Firstly, the chemical composition of the shaft is done by an Optical Emission Spectroscopy (OES) method, the found chemical composition was matching with required standard value. Mechanical testing consists of two test i.e. tensile test and hardness test and it was found out that the strength and hardness of specimens were within the required capacity. For metallurgical analysis, the microstructure of the shaft was developed by using an optical microstructure. Equal distribution of ferrite perlite shows that heat treatment was performed well and carbon percentage in a material is satisfying the standard values. Thus, it proves that the material used was of good quality and indicates that failure is not due to material property. Further for the fractography, the fractured surface was examined by SEM. The cross-section was taken in a quarter segments and divided into four regions i.e. A, B, C, and D. Fractography morphology mainly showed that the failure of roller shaft was caused due to fatigue. To examine the stress distribution at the fractured surface the Finite Element Method (FEM) was also carried out. Based on the shaft size, a precise ANSYS model was developed. The result of FEM shows that stress concentration was significant at roller shaft step which could reduce the material reliability to some extent. Based on the failure analysis it could be concluded that due to stress concentration a micro crack is initiated along the weak interface and further it converted into the major fatigue failure. Fractography morphology of failed roller shaft also confirms the fatigue failure.

Through a field failure investigation and finite element analysis, a study on overheated pipe due to scale was performed. The field failure samples were collected from primary reformer convection tube made of material based on ASTM A 335 grad P91 modified specification. The type of the object was a fin tube coil. The tube shows rupture with the shape of fish mouth accompanied by bulging. The analysis performed using finite element mimics the actual field conditions. It was discovered that long term overheating due to scale both at the outside and in the inside of the tube hinders the smooth heat transfer process. This consequently leads to poor heat transfer and violates the original design philosophy. Metallurgical evaluation proves this verdict. Furthermore, the finite element analysis also supports the conclusion.

Various degradation mechanisms are characterized in gas turbine rotor blades due to service conditions such as: high temperature and stress. Failure of turbine blade can have the tremendous effects on the safety and performance of the gas turbine engine. This paper investigates a first stage turbine blade failure in a 6.5 MW gas turbine. The blade is made of nickel-based superalloy, and the failure occurred in the airfoils after 6500 h of operation. Several examinations were carried out in order to identify potential failure reasons such as: visual examination, fractography and microstructural characterization used by optical and scanning electron microscopes (SEM) and energy dispersive X-ray (EDX). The precipitated phases morphology (carbides and γ′ (Ni3Al)) changed in the airfoil for example γ′ resolved and re-deposited in addition to decomposition of carbides. Furthermore, the fracture surface exhibits the local melting occurred and re-solidified in the leading edge. From analysis and experimental results of this study, overheating is shown to be the main reason of blade failure.

Regardless of their specific applications, all the vehicles used in mining operations are subjected to severe working conditions that reduce in a considerable amount, their active in-service life. In this work, the causes that promote failure of the fasten system and subsequent ejection of the wheels of passenger pickup trucks used in open-pit mines are analysed. By means of scanning electron microscopy, optical microscopy analyses and hardness tests, it was found that failure of the fasten system is characterised by a series of synergetic steps that include, the plastic deformation of the lug nuts caused by deficient tightening practices, fatigue and plastic deformation of the bolts. When combined, these phenomena leaded to the formation of cracks that propagated in the radial direction of these elements. The reasons that promote the development of this kind of failure are presented and discussed in this investigation.